Whole virus vaccines administered to the body elicit an immune response by the formation of antibodies to the viral antigen. In the case of the influenza virus, it is known that the influenza HA protein is the target of virus neutralizing antibodies for this virus. One commercially-available whole virus vaccine is a split virus vaccine, obtained by treating inactivated virus with detergent, is sold under the trade-mark FLUZONE.RTM. by Connaught Laboratories, Inc.
The trend with respect to vaccines is away from whole virus materials and towards more purified materials, which generally are smaller and well defined. The influenza HA antigen has been isolated but the specific subunit materials are only weakly immunogenic and are incapable of inducing a sufficiently high immune response to be effective in many classes of individuals.
There was previously described in published EP 87-310377, assigned to the assignee hereof, the covalent bonding of HA to diphtheria toxoid. As described therein, the HA was removed from the whole virus by bromelain cleavage and the resulting HA-subunit was covalently bonded to diphtheria toxoid using a heterobifunctional cross-linker of the maleimide-N-hydroxy-succinimide ester type, particularly MCS (maleimido-caproic acid-N-hydroxysuccinimide ester). In preparing the HA-D conjugate, a sulfhydryl group first is introduced to the HA-subunit by treatment with SATA (N-succinimidyl-S-acetylthioacetic acid), the N-hydroxysuccinimide ester is reacted with amino groups on the diphtheria toxoid (DT), and then the maleimide component is reacted with the free sulfhydryl groups introduced to the HA antigen to link the HA and DT molecules to form the HA-D conjugate.
While the results which were obtained in human efficacy trials showed the HA-D conjugate vaccine to produce a greater immune response than the HA subunit vaccine alone, the HA-D conjugate was not more effective in producing an immune response in humans, when compared to whole influenza virus vaccine.